1. Field of the Invention
This invention relates to thermoplastic polymers having improved impact modification, processes for making such polymers, and products made from such polymers.
2. Description of the Related Art
Thermoplastic olefins (TPOs) are generally produced from blends of an elastomeric material such as ethylenelpropylene rubber (EPM) or ethylene/propylene diene monomer terpolymer (EPDM) and a more rigid material such as isotactic polypropylene. Other materials or components can be added into the formulation depending upon the application, including oil, fillers, and cross-linking agents. Generally, TPOs are characterized by a balance of stiffness (modulus) and low temperature impact, good chemical resistance and broad use temperatures. Because of features such as these, TPOs are used in many applications, including automotive facia and wire and cable operations.
It is well known that narrow molecular weight distribution linear polymers disadvantageously have low shear sensitivity or low l.sub.10 /l.sub.2 value, which limits the extrudability of such polymers. Additionally, such polymers possess low melt elasticity, causing problems in melt fabrication such as film forming processes or blow molding processes (e.g., sustaining a bubble in the blown film process, or sag in the blow molding process etc.). Finally, such resins also experienced surface melt fracture properties at relatively low extrusion rates thereby processing unacceptably and causing surface irregularities in the finished product.
While the development of new lower modulus polymers such as Flexomer.TM. polyolefins by Union Carbide or Exact.TM. polymers by Exxon has aided the TPO marketplace, there continues to be a need for other more advanced, cost-effective polymers for compounding into polypropylene which improve or maintain low temperature impact performance, elongation to break, modulus, tensile strength and weld line strength. These and other needs are met by the present invention.